Since optical discs such as a CD (Compact Disc) and a CD-ROM (Compact Disc Read Only Memory) are easy to handle and are produced at relatively low cost, they have been widely used as recording mediums for storing data. In recent years, a CD-R (Compact Disc Recordable) disc, on which data can be recorded once, and a CD-RW (Compact disc ReWritable) disc, on which data can be rewritten, have come out. Thus, data can be more easily recorded on such optical discs than before. As a result, optical discs such as a CD-DA (Compact Disc Digital Audio) disc, a CD-ROM disc, a CD-R disc, and a CD-RW disc have become the mainstream of data recording mediums. In addition, in recent years, audio data is compressed according to compression-encoding systems such as the MP3 (MPEG1 Audio Layer-3) and the ATRAC (Adaptive TRansform Acoustic Coding) 3 and recorded on the CD-ROM disc, the CD-R disc, the CD-RW disc, and so forth.
However, as the CD-R disc and the CD-RW disc have come out, data recoded on the CD-DA disc can be more easily copied than before. As a result, a problem about copyright protection has arisen. Thus, when content data is recorded to the CD-R disc or the CD-RW disc, it is necessary to take measures to protect content data.
As one method for protecting content data recorded on the CD-DA disc, the content data is encrypted and recorded on the CD-DA disc. When content data is encrypted and recorded on a disc, unless key data with which the content data is decrypted is obtained, the content data cannot be decrypted. Thus, the content data can be protected.
However, when encryption key data and a disc are separately distributed, a particular system is required and becomes troublesome. Thus, it is preferred to bury encryption key data on a disc. However, it was difficult to record encryption key data on a disc in such a manner that the encryption key data cannot be easily known. That is because it is necessary to prevent buried encryption key data from adversely affecting a conventional CD player, which has been widely used.